I don't know whether this has been said by someone already. If that's the case, I apologise. But I just thought I'd share my recent discovery
I really dislike the standard font used on the sony readers. I dislike it so much I haven't really used my Sony the last couple of months, instead doing all my reading on my Kindle. I tried changing the font a couple of times but for some reason never succeeded.
But now I've found a really easy way to convert books into a rather pleasant sand-serif font, and it's super easy. You just type in *{font-family: sans-serif;} in the Extra CSS box when you convert a book in Calibre. I bulk-converted all my books that way.
So if you want a font that's easier to read but don't want to work too hard to get it, this is the way to go!